This application relates to a seismic sea bed source arranged for generating shear waves.
More particularly it relates to a device for coupling a shear wave generator in a releasable way to a sedimentary sea bed.
U.S. Pat. No. 4,705,137 xe2x80x9cMarine Shear Vibratorxe2x80x9d describes a subsea vibrator apparatus for generating a shear wave into the consolidated sea bed, with a house, a horizontally arranged mass attached to the house for movement along an axis being mainly parallel to the earth""s surface, and hydraulic apparatus for oscillating the mass along the axis. In essence the entire vibrator house comprises a sidewall and a bottom. The sidewalls and the bottom form a tapered surface with an angular surface of the tapered surface transversal to the mass""movement, so that the mass oscillates the subsea vibrator against the consolidated bottom, where the vibrator will sink and settle into the sea bed. However, the vibrator will maintain contact with the bottom even though the consolidated bottom yields under the vibrated weight.
U.S. Pat. No. 5,128,900 describes a method and an apparatus for generating elliptically polarized shear waves in marine environments with a shear wave source penetrating surface sediments in the sea bed and comprises a double actuator/reaction mass system mounted inside a structural ring of a conical house. A cross structure on top of the apparatus serves as a passive reaction mass and as a moment arm support for reversibly motor driven propellers in sleeves arranged at the outer ends of the cross structure. The cross structure and the house is connected with a structurally rigid member like a vertical pipe. Pistons provide actuation of two reaction masses inside the house, and the movements of the reaction masses generate shear movements propagating into the subsurface formations.
Two essential problems are related to the known art:
a) To obtain sufficient coupling between the shear wave source and the consolidated sediments. This is Particularly related to soft sediments in the sea bed surface, with the sediments constituded by organic sediments, gyttja, mud, unconsolidated clay and other unconsolidated sediments having very low density compared to the underlying more consolidated sedimentary sea bed. This incurs an inferior transmission of the vibrators movements into seismic waves.
b) To release the shear wave source from the consolidated sediments. U.S. Pat. No. 4,705,137 will have a large surface which easily will stick firmly in the sea bed after vibration, requiring large forces to release from the sea bed. U.S. Pat. No. 5,128,900 will incur a risk of becoming stuck in the sea bed if it obtains a good fix, and may sink deeper into the sediments during vibration.
c) To achieve a uniform coupling to the sea bed during the entire vibration time. The two U.S. patent may sink deeper into the sediments during the seismic signal generation because they are given a wedge-shape penetrating into the sea bed and with vibration both being a means for attaching the signal source and also for generating the seismic signal. This may incur uncertainty during the interpretation of the seismic signal because one may not, to a sufficient degree, control the seismic characteristics of the source.
A solution to the above mentioned problems is a seismic shear wave generator comprising a seismic source arranged on a base plate, for being arranged in a working position at the sea bed, and a coupling device arranged for coupling the wave source mechanically to the sea bed so that shear waves can be transmitted to the sea bed, with the coupling device comprising a cylindrical skirt arranged closing along the periphery of the base plate and extending downwards from the underside of the base plate, and open downwards, and a pumping device arranged for pumping water out of, or into the skirt, respectively, through the base plate to form underpressure, or possibly overpressure in a closed cavity being delimited by the skirt, the base plate and the sea bed, when the skirt is lowered to or entirely or partially penetrated into the sea bed.